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arXiv · 2509.07077

Cosmology, Decoherence and the Second Law

Sebastian Cespedes·Senarath de Alwis·Fernando Quevedo
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Abstract

We consider quantum decoherence and entropy increase in early universe cosmology. We first study decoherence in a discrete bipartite quantum system for which a single qubit gets entangled with an environment and the entropy increase is correlated with the decay of the off-diagonal terms of the reduced density matrix. We compare this system with continuous systems relevant for cosmology for which there is a natural external intervention, corresponding to the time-dependent separation between the sub- and super-horizon inflationary fluctuations. We find, in this case, that the off-diagonal terms of the density matrix, in a field basis, do not decay as sometimes assumed in cosmological set-ups. Nevertheless, following a recent treatment in terms of open Effective Field Theories (EFTs), we compute the entanglement entropy for a Gaussian state and show that it actually increases monotonically ($\dot S>0$) during the accelerated phases ($\ddot a>0$ with $a(t)$ the scale factor). We generalise this result to include non-Gaussian states and briefly discuss the relevance of computing the von Neumann entropy as compared to the thermodynamic entropy.

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Sebastian Cespedes, Senarath de Alwis, Fernando Quevedo. 2025-09-08. Cosmology, Decoherence and the Second Law. https://arxiv.org/abs/2509.07077

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